It all depends on what geography your rainfall data were collected in (i.e. zip code, county, some other boundary). You will need to get a shapefile/feature class of the boundaries in the map. Once you have this, you can add the Excel document to the map. Then, you will need to do a table join to get the data from the Excel spreadsheet into the shapefile. You can then symbolize your shapefile based on the data in your Excel spreadsheet.